The development of the hypothalamo-pituitary axis in the neonatal rat: pituitary and serum prolactin concentrations.

Using highly specific double-antibody radioimmunoassay techniques, we have measured pituitary and serum prolactin (PRL) concentrations in neonatal rats from birth to 60 d. Pituitary PRL concentration was low at birth in both sexes and peak prepuberal values were attained at 25-28 d. Following this time, pituitary PRL concentrations in females were significantly higher than those observed in males (p less than 0.01). Serum PRL concentrations were low at birth in both sexes and remained low until 15 d, when they rose to peak values just before puberty. There were no significant sex differences with respect to serum PRL concentrations. These data indicate that hypothalamic control of PRL secretion developes during the neonatal period of the rat. The relationship of this developmental process in the neonatal rat as a possible model for the study of the midgestational hypothalamic maturation of the human fetus is discussed.